I normally have my Engel set at 2-3 degrees and it seems good at that even in 35 degree heat.
Another thing I sometimes do is switch it off before going to bed and then switch it back on in the morning. If its a cold night it only seems to climb a few degrees.
I read a while ago that a full fridge is always more efficient than a half full/empty one and to try and limit how often you open the fridge and let all that nicely cooled air out. So grab all the food out for dinner in one go, or if your getting a drink ask the rest of the family if they want one at the same time etc.
I find mine will run quite happily for 3-4 days running constantly off the 2 batteries, but I'm keen to get a solar panel so I can camp for an unlimited amount of time without running the engine.
Depending on the sort of trips you plan to do, might influence whether you decide to have the fridge bolted in the back of your 80, on a fridge slide for example, or just held in place with straps.
Mine is bolted to my slide which is fine most of the time, but if I was camping in one spot for say a week, it might be nice to be able to lift the fridge out and leave that at the camp with the Mrs while I go off for a drive. Being bolted in makes it much harder to do that.